Comparing the efficacy of thyroglobulin and thyroglobulin/ thyroid-stimulating hormone ratio models in predicting a successful response to radioactive iodine therapy

Background The thyroglobulin (Tg)/ thyroid-stimulating hormone (TSH) ratio has manifested to be a reliable marker for predicting prognosis in patients with differentiated thyroid carcinoma (DTC). The objective of this study was to compare the efficacy of Tg and Tg/TSH ratio models in predicting a successful response to radioactive iodine therapy. Methods One thousand six hundred forty-two DTC patients receiving 131I radiotherapy were finally enrolled in this retrospective study. The patients were divided into a training set (n = 973) and a validation set (n = 669) by the patient consultation time (July 2019). A receiver-operating characteristic curve was constructed for Tg and the Tg/TSH ratio to establish their cutoffs. Then, the variables were screened by univariate logistic regression and incorporated into logistic prediction models by stepwise regression, where Tg/TSH was excluded from model 1 and Tg was excluded from model 2. Results In 1642 enrolled DTC patients, the first 131I radiotherapy had an excellent response in 855 patients. The cut-offs for Tg level and Tg/TSH ratio were 3.40 ng/ mL [area under the curve (AUC): 0.789] and 36.03 ng/mIU (AUC: 0.788), respectively. In addition, the AUC of the model including Tg was higher than that of the model including Tg/TSH in both the training set (0.837 vs 0.833) and the testing set (0.854 vs 0.836). Conclusions Both Tg and Tg/TSH ratios could be considered predictors of the effects of the first 131I ablative therapy. However, the prediction model including Tg performed better than the model including Tg/TSH.


Introduction
Thyroid cancer ranked 9th for incidence in 2020 and is the most common endocrine malignancy worldwide [1,2]. Of all the thyroid carcinoma, differentiated thyroid cancer (DTC) accounts for nearly 90% [3]. The standard treatment for DTC is total or near-total thyroidectomy followed by radioactive iodine (RAI) therapy and thyroid-stimulating hormone (TSH) suppression with levothyroxine (L-T4). With this treatment approach, the majority of patients with DTC could get improved disease-free survival and long-term prognosis [4]. However, in some patients, the persistent disease develops after initial treatment, or cancer recurrence is detected during follow-up.
The RAI could destroy the remaining residual tissue or metastatic tissue susceptible to radioiodine uptake, and is crucial to the treatment of thyroid cancer [5]. Its results affect the prognosis of patients with DTC and guide the decision-making process leading to RAI treatment. Thyroglobulin (Tg) has been demonstrated to be critical in predicting therapeutic response to RAI [6][7][8]. Since the production of Tg could be stimulated by the TSH, Tg values may be affected by the TSH values of pre-ablative patients. For this reason, the relationship between the Tg/ TSH ratio and the outcome of the first 131 I radiotherapy has been extensively studied [9][10][11]. However, few studies have compared the predictive model capability to the RAI ablation therapy of Tg and Tg/TSH. This study aimed to assess the factors affecting the outcome of the first 131 I radiotherapy in patients with DTC and compare the efficacy of Tg and Tg/TSH ratio models in predicting a successful response to radioactive iodine therapy.

Study patients
This retrospective study reviewed the clinical records of 2331 DTC patients treated consecutively at our institution between September 2015 and September 2020. Patients' data were extracted from the clinical electronic medical record system of the Second Hospital   of Shandong University. Inclusion criteria were as follows:(1) adult patients with anatomopathological DTC diagnosis, (2) patients who received total or near-total thyroidectomy, and (3) patients who received 131 I treatment followed. Exclusion criteria were as follows:(1) patients who presented thyroglobulin antibody (TgAb) ≥ 115 IU/ml, (2) patients who lost to follow-up and had inadequate information (Fig. 1). After the inclusion and exclusion criteria screening, 1642 patients were finally enrolled in this study. Patient data extracted from the clinical electronic medical record system was identified such that all private information was not included. The informed consent was waived by the Institutional Review Board of the Second Hospital of Shandong University [KYLL-2018(LW)013].
All procedures complied with the Declaration of Helsinki for research involving human subjects.

Treatment and examination
All patients underwent L-T4 withdrawal or no L-T4 therapy after total or near-total thyroidectomy for 3-4 weeks, accompanied by a low-iodine diet (< 50 ugs per day) for 1-2 weeks. When prepared well, a treatment dose from 1.11 GBq (30 mCi) to 5.55 GBq (150 mCi) of 131 I was given on an empty stomach according to their tumor stage and recurrence risk stratification. The following tests were completed within three days before 131 I treatment, including TSH, free thyroxine, free triiodothyronine, Tg, and TgAb. analysis classified the four response-to-therapy categories into ER and non-excellent response (NER), the latter including IDR, BIR, and SIR.

Statistical analysis
Continuous variables were reported as mean and standard deviation or median values and quartiles, while categorical variables were reported as numbers and percentages. Between groups, differences were assessed with  the student t-test (and Mann-Whitney U test) or the chi-square test (categorical variables). The patients were divided into a training set (n = 973) and a validation set (n = 669) by the patient consultation time (July 2019). A receiver-operating characteristic (ROC) curve was constructed for Tg and the Tg/TSH ratio to establish their cutoffs. Then, univariate logistic regression was used to screen the factors associated with RAI therapy efficacy. The screened variables were incorporated into logistic prediction models by stepwise procedure, where Tg/TSH was excluded from model 1 and Tg was excluded from model 2. The performance of the models was evaluated by the AUC, accuracy, and specificity. The accuracy and clinical benefit of the model were assessed by the calibration curves and decision curves, separately. All tests were two-sided, and a P-value < 0.05 was considered statistically significant. The statistical analysis was performed on the software package R (version 4.1.1).

Characteristics of patients
Of the 1642 DTC patients receiving 131 I thyroid remnant ablation, the mean age at diagnosis was 43.69(± 11.50) years, and the majority (n = 1162; 70.8%) of DTC were female. Characteristics of study subjects are presented in Table 1.

Comparison between excellent response and non-excellent response groups
In 1642 enrolled DTC patients, the first 131 I thyroid remnant ablation had an excellent response in 787 patients and had an incomplete answer in the remaining 855 patients, with a rate of excellent response of 52.1%. The differences in gender, metastasis number, benign lesion, ATA risk stratification, Tg and Tg/TSH ratio between patients with successful and unsuccessful ablation were statistically significant. (P < 0.05) ( Table 2).

Analysis of the factors affecting the efficacy of 131 I radiotherapy
Univariate analysis showed that the outcome of 131 I radiotherapy was influenced by age, gender, metastasis number, with or without benign lesion, Tumor-nodemetastasis (TNM) stage, ATA risk stratification, pretreatment Tg level and Tg/TSH level (Fig. 2). According to the ROC curve, the cut-offs for the Tg level was 3.40 ng/ mL [area under the curve (AUC): 0.789] with 63.4% sensitivity and 84.0% specificity, while for Tg/TSH ratio was 36.03 ng/mIU (AUC: 0.788) with 67.2% sensitivity and 81.5% specificity (Fig. 3).
Age, sex, ATA risk stratification and Tg level were all independent influencers of RAI success in model 1, and age, sex, ATA risk stratification and Tg/TSH level were all independent influencers of RAI success in model 2 (P < 0.05) ( Table 3). The AUC of model 1 was higher than that of model 2 in both the training set (0.837 vs 0.833) and the testing set (0.854 vs 0.836) ( Table 4). Besides, both the ROC curve and decision curve of model 1 in the validation set were significantly higher than that of model 2 (Fig. 4).

Discussion
Unsuccessful ablation may adversely affect the prognosis of patients with DTC; therefore, predicting the outcome of RAI treatment is important to the treatment and follow-up protocols of patients with DTC. Tg is a glycoprotein that is explicitly produced by thyroid cells and has been demonstrated as a prognosis predictor implying 131 I therapy response before RAI [12][13][14]. Since TSH is the trigger factor that directly affects the expression of Tg [15], many have studied the Tg/TSH ratio in addition to Tg to investigate the predictive value for RAI therapy in patients with DTC [16,17]. Some studies have reported that both Tg and Tg/TSH ratios could be considered predictors of the effects of RAI therapy [9,18].
In this study, the cutoffs were 3.40 ng/ mL for Tg and 36.03 ng/mIU for the Tg/TSH ratio in predicting successful RAI according to the ROC curve. Though the cutoffs differed from the results of previous studies due to various measurement kits with different sensitivities and display units, this study reached similar findings to some studies that the Tg/TSH ratio and Tg were equally reliable in predicting successful RAI in DTC patients [9,18]. However, Other studies had different views on the comparison of Tg and Tg/TSH. Some researchers have found that Tg/TSH ratio had a stronger association with ablation outcome than Tg [10]; While others considered that the predictive value of the Tg/TSH ratio was inferior to the prognostic value of Tg alone [19]. The comparison of the Tg and Tg/TSH ratio in predicting the outcome of RAI treatment remained controversial. To further compare the predictive performance of Tg and Tg/TSH to the response of the RAI therapy, we constructed 2 predictive models and compared them in the training and validation sets respectively. The study showed that in the testing set, the AUC of the predictive model including Tg to predict successful ablation was 0.854 with 75.3% sensitivity and 82.3% specificity, while the AUC of the predictive model including Tg/TSH was 0.836 with 74.7% sensitivity and 79.2% specificity. In addition, the ROC curves and decision curves also showed that the predictive model including Tg performed better in predicting the results of RAI therapy in patients with DTC. Thus, in the present study, the model including Tg had greater predictive accuracy and higher clinical benefit than the model including Tg/TSH. This showed that Tg was a better predictor of the DTC effect than Tg/TSH in the clinical setting.
Compared with other studies, our study included a relatively large number of participants, the outcome of each participant was observed by ourselves, and there was almost no recall bias; In addition, we also conducted a sensitivity analysis by dividing the training set and the validation set to verify the performance of the model, and the research results are convincing. There are several limitations to this study. First of all, the data about TSH tests at several time points could not be analyzed in this study, which was not available in routine practice. Besides, this study had a relatively short follow-up time, and the longterm clinical implication of the Tg and Tg/TSH needs to be assessed by additional prospective studies.

Conclusion
In conclusion, the present study found that both the Tg and the Tg/TSH ratio could affect the effects of RAI therapy, and the prediction model including Tg performed better than the model including Tg/TSH. Consequently, Tg was more recommended as the factor to predict the efficacy of RAI therapy given its better predictive efficacy in this study.